After just 1 hour, they found each compound had sunk into the skin, meaning its protective effect was greatly reduced.

By New Scientist ,WHEN out in the sun, how often do you apply sunscreen? If it's anything less than once every 2 hours, you might be better off not using any in the first place.

So says Kerry Hanson, a chemist at the University of California at Riverside. She and her colleagues exposed human skin samples grown in the lab to UV radiation while they were covered with three common UV filters found in sunscreens: benzophenone-3, octocrylene and octylmethoxycinnamate. After just 1 hour, they found each compound had sunk into the skin, meaning its protective effect was greatly reduced. Worse, Hanson's team found that the samples contained more reactive oxygen species (ROS) than skin exposed to UV with no sunscreen on it. ROS are free radicals that can damage skin cells and increase the risk of skin cancer (Free Radical Biology and Medicine, DOI: 10.1016/j. freeradbiomed.2006.06.011).

The Skin Cancer Foundation in New York recommends that people go no more than 2 hours between reapplications of sunscreen. Our findings tend to support that, says Hanson.

It might actually be necessary to reapply even more often. One way of counteracting free radicals, Hanson says, might be to add antioxidants such as vitamins C and E to sunscreens. "In previous work, we've shown that antioxidants can help neutralise ROS in the skin," she says, though she has yet to perform the same experiment with sunscreen.

The notion that sunscreen can increase damage to skin caused by UV rays is startling, says a spokeswoman for the Skin Cancer Foundation. "It's a very strong statement they're making." The take-home message? Reapply regularly.

- THIS ARTICLE APPEARS IN NEW SCIENTIST MAGAZINE ISSUE: 9 SEPTEMBER 2006